JSFiddle - React, Tailwind, and code Playground

HTML

<select class='parent' name='parent' size='10'>
    <option value='1'>Category 1 -></option>
    <option value='2'>Category 2 -></option>
</select>

<select class='child' id='child' size='10'>
    <option class='sub_1' value='5'>Custom Subcategory 1.1</option>
    <option class='sub_1' value='3'>Subcategory 1.1</option>
    <option class='sub_2' value='4'>Subcategory 2.1</option>
</select>

JavaScript

$(function() {
    $('#child').change(function() {  $('.parent').val($(this).children(':selected').attr('class').substr(4));
    });
});